1. Field of the Invention
The present invention relates to a zoom optical system which is used in an optical apparatus such as an image projecting apparatus and an exposure apparatus.
2. Related Background Art
Some of the optical apparatuses described above require a zoom optical system excellent in a telecentric characteristic on the object side and excellent in invariance of the positions of an object surface, an image plane and an exit pupil against variations in the focal length.
For example, Japanese Patent Application Laid-Open No. 2002-207167 discloses a zoom optical system in which a lens unit having a negative refractive power or a lens unit having a lens element having a strong negative refractive power is moved toward the object side as the focal length decreases, whereby the positions of an object surface, an image plane, an entrance pupil and an exit pupil are made invariant against variations in the focal length, as a zoom optical system suitable for an illumination optical system mounted on a projecting exposure apparatus.
It is conceivable that the zoom optical system disclosed in Japanese Patent Application Laid-Open No. 2002-207167 is used for a projecting optical system of an image projecting apparatus projecting on a magnified scale an image of an original formed on a liquid crystal panel, or a part thereof.
However, if the liquid crystal panel is disposed on a reduction-side focal plane of the zoom optical system in Japanese Patent Application Laid-Open No. 2002-207167, a sufficient back focus cannot be obtained because in the zoom optical system, a lens unit in the most magnification side (aperture stop side) has a positive refractive power. In so called a tri-panel type image projecting apparatus, a color synthesizing member synthesizing chromatic light of three colors of red, green and blue is disposed on the reduction side in the zoom optical system, but if a sufficient back focus is not obtained as described above, it is difficult to provide space for displacing the color synthesizing member.
In the zoom optical system disclosed in Japanese Patent Application Laid-Open No. 2002-207167, a lens unit in the most reduction-side has a negative refractive power, and as the focal length decreases, a lens unit disposed in the most magnification side than the negative lens unit moves toward the magnification side, thus increasing the interval between both lens units. As a result, the effective diameter of the positive lens unit increases. Further, because the lens unit in the most reduction side has a negative refractive power, the numerical aperture (NA) on the reduction side decreases.
Japanese Patent Application Laid-Open No. 2002-207167 discloses the case where a lens unit in the most reduction side has a positive refractive power, however this lens unit considerably moves toward the magnification side as the focal length decreases, and therefore if the NA on the reduction side is increased, the effective diameter of the lens unit increases.
Namely, in this zoom optical system, a plurality of lens units are situated in the aperture stop side in a state of the minimum focal length (wide), and move toward the reduction-side focal plane side as the focal length increases, the effective diameter of the lens unit therefore tends to increase, and it is thus difficult to make the zoom optical system compact. Further, because the lens unit in the most magnification side has a positive refractive power, it is also difficult to provide a sufficient back focus. Moreover, the zoom optical system is based on a single-wavelength light source, and is not suitable for projection display of color images.